Radio-frequency identification (RFID) of tagged objects is a promising technology with potentially broad applications in commerce, livestock management, wildlife management and security. Currently, RFID tags are replacing bar codes in commercial applications for purposes of inventory tracking and retail check-out. Generally, an RFID system consists of an RFID tag-reading instrument and one or more RFID transponder tags. In some embodiments, the transponder tags are compact, microfabricated, backscatter tags, e.g., tags that do not require an on-board source of power, which respond to a query or interrogation signal broadcasted by the tag reader. These tags can harvest some of the energy from the broadcasted query for on-board processing, and then scatter back some of the incident energy to communicate information, e.g., the identity of the tag, to the tag reader. Because of their potential small size and low cost, RFID tags may be attached to, or placed with, a wide variety of objects such as commercial goods, vehicles, livestock, wildlife, and identification cards or bracelets. Since each RFID tag can be given a unique identification number, an RFID tag reader can quickly detect and record the presence of each RFID transponder tag within the broadcast-and-receive range of the tag reader. Accordingly, an RFID system can be used to inventory goods and livestock, or determine the presence and identity of wildlife or subjects carrying RFID tags.
Although RFID technology is advancing, there have been limited developments in adapting RFID technology to determine tag range, e.g., distance between a particular tag and a tag reader's antenna, or tag location. An embodiment of RFID technology adapted for tag range determination can be found in U.S. Pat. Nos. 7,119,738 and 7,030,761.